1. Field of the Invention
This invention is related to the field of acoustic actuators and acoustic control, and in particular, relates to an acoustic actuator mounted in a small sealed enclosure having an extremely low natural frequency.
2. Description of the Prior Art
Often, it is necessary to mount acoustic actuators (such as loudspeakers, i.e., speakers) in relatively small enclosures. This may result from constraints on the space available for mounting or in order for the device to be unobtrusive. However, when a speaker is mounted into a small sealed enclosure, the natural frequency (the fundamental mechanical resonance frequency) is higher than would be exhibited in a large enclosure system due to the air spring effect within the enclosure. This degrades the low frequency performance of the speaker, since speakers radiate sound less effectively below their natural frequency. Consequently, speakers designed for low frequency use are often mounted in very large enclosures. Large volume enclosures have less of an air spring effect, which allows for a lower resonance, improving the speaker""s low frequency performance.
If an active feedback circuit is implemented to regulate the motion of the diaphragm, the loudspeaker""s natural frequency can be tuned to a specific frequency. Such feedback loops have been widely used to control diaphragm motion and create speaker systems with enhanced low frequency performance (see U.S. Pat. Nos. 5,086,473, and 5,588,065). In addition to audio applications, motion controlled speakers have also been successfully implemented in noise control applications where the low frequency response of the actuator is crucial (U.S. Pat. No. 5,771,300). However, this technique has the disadvantage of requiring active feedback circuitry and additional power sources, and may not be practical to implement.
The stiffness contribution from the air volume that occurs when mounting acoustic actuators in small sealed enclosures can significantly increase the speaker system""s natural frequency. As a result, the actuator is less effective at low frequencies. The present invention is an acoustic actuator mounted in a small sealed enclosure having an extremely low natural frequency. This is accomplished in part by partially or completely evacuating the air from the enclosure to minimize the air spring effect. Additionally, a buckling suspension system is utilized that exhibits a non-linear spring rate. The suspension is capable of supporting the large loads associated with the pressure differential across the diaphragm while simultaneously exhibiting a low stiffness on the order of conventional speaker suspensions. The present invention enables the construction of small-enclosure-mounted sub-woofers that perform as well or better than similar large-enclosure-mounted sub-woofers. It is also applicable to noise control systems where low frequency response is critical, e.g., small-volume passive acoustic attenuators for damping acoustic modes in rooms.